Friction material and manufacturing method thereof

ABSTRACT

A friction material is provided with a matrix fiber, a binder, and a filler. The friction material contains a stabilized zirconia having a lattice constant in a range from 99.93 to 99.95% as compared with a lattice constant of a single crystal of stabilized zirconia. The lattice constant scarcely changes, since the stabilized zirconia contained in the friction material undergoes less introduction of distortions when used under severe conditions. Accordingly, the stable friction characteristic with less change of the friction coefficient in the initial stage of using the brake can be obtained.

This application claims foreign priority from Japanese Patent Application No. 2005-379127, filed Dec. 28, 2005, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a friction material of a brake and a clutch, used, for example, in automobiles and industrial machines, and a manufacturing method thereof. More specifically, the invention relates to a friction material with a scarce change in the friction coefficient and capable of ensuring a stability of the friction coefficient at the initial stage of using new friction materials, as well as a manufacturing method thereof.

2. Related Art

In recent years, along with improvement in the performance and increase in the power of automobiles, thermal stability has been demanded also for a friction material such as a brake lining used, for example, in drum brakes, disc brakes, etc. of automobiles. Specifically, mechanical strength, structural stabilities and wear-resistance at elevated temperatures are required, as well as stable friction characteristics such as less change of friction coefficient during a frequent change of temperatures have been demanded.

Generally, the friction material is composed of various ingredients. That is, they include matrix fibers such as glass fibers and aramid fibers, lubricants such as graphite and molybdenum disulfide, fillers, and binders such as phenolic resin.

Further, as the fillers, materials adjusting friction characteristics such as organic dust, metal powder, inorganic compound have been used in addition to the lubricant such as graphite and molybdenum disulfide. As other fillers, materials so-called abrasives such as alumina having 7 or more Mohs hardness, i.e., are used for increasing the friction coefficient of the friction materials high.

In addition to the abrasives described above, those capable of remarkably raising the friction coefficient of the friction materials include monoclinic zirconia. For example, JP-A-62-020581 discloses a friction material composition, for use in a brake lining, containing a zirconium oxide powder by 0.5 to 10% by weight. JP-A-03-185030 discloses an on-asbestos type friction material incorporated with zirconium oxide as at least one kind of fillers. Further, abrasives improving the friction characteristic at high temperature include stabilized zirconia. For example, JP-A-09-031440 discloses a friction material containing zirconium oxide in which the zirconium oxide is stabilized with at least one of calcia (CaO), yttria (Y₂O₃), and magnesia (MgO). JP-A-2000-160135 discloses compositions of friction materials containing 1 to 20% by weight of partially stabilized or fully stabilized zirconium oxide having a Mohs hardness of 7 or more and an average particle size of from 0.2 to 70 micron (μm). JP-A-09-031440 discloses that the amount of wear can be reduced by using stabilized zirconia instead of unstabilized monoclinic one. JP-A-2000-160135 discloses that it is possible to prevent generation of uncomfortable sound called as groan by the use of the monoclinic zirconia. In each of them, the problem caused by structural phase transition of monoclinic zirconia at high temperatures (800 to 1200° C.) from a monoclinic crystal phase to a tetragonal one is solved by the use of stabilized zirconia which does not cause any structural phase transitions and, therefore, maintains cubic crystals at the high temperatures.

However, in the existent friction materials using the stabilized zirconia, after a brake is used under conditions at which so-called “fading phenomena” occur or under other high load conditions at an initial stage of using a new friction materials, the friction coefficient changes greatly to result in a problem such as occurrence of various kinds of noise. Although to improve the problems, various studies on the amount of addition, particle sizes, and/or stabilized ratio of stabilized zirconia have been attempted in the prior art, the problems have not been solved till now.

SUMMARY OF THE INVENTION

In view of the foregoings, the present invention intends to provide a friction material having stable friction characteristics such as less change of friction coefficient at the initial stage of using new friction materials even after the friction materials were used under severe braking conditions in which such as fading phenomena occur.

The inventors have noted on the lattice constant of stabilized zirconia contained in the friction materials which had not considered in the above-mentioned Japanese Patent publications and the values of the lattice constant also thereof have not yet been analyzed. As a result of their studies, it has been found that the lattice constant of stabilized zirconia contained in the friction material changed greatly before and after the use of the friction material. Then, the invention has been accomplished based on the finding that the friction coefficient at the initial stage can be stabilized with a friction material containing stabilized zirconia having a lattice constant within a predetermined range. That is, in accordance with one or more embodiments of the present invention, a friction material is provided with a matrix fiber, a binder and a filler and utilizing a stabilized zirconia having a lattice constant in a range from 99.93 to 99.95% as compared with a lattice constant of a single crystal of stabilized zirconia. The stabilized zirconia having the lattice constant in the range from 99.93 to 99.95% as compared with that of the single crystal of stabilized zirconia may be a calcia-stabilized zirconia having a lattice constant of from 5.1253 to 5.1263 Å. Further, in accordance with one or more embodiments of the present invention, a friction material is provided with a matrix fiber, a binder, and a filler in which a stabilized zirconia having a lattice constant in a range from 99.95 to 99.97% as compared with a lattice constant of a single crystal of stabilized zirconia is used as a compound of the filler. The stabilized zirconia may be a calcia-stabilized zirconia having a lattice constant in a range from 5.1263 to 5.1275 Å. Further, in accordance with one or more embodiments of the present invention, a method of manufacturing a friction material is provided with steps of uniformly mixing a matrix fiber, a binder, and a filler, and molding a mixture of them under heating and pressing, in which a stabilized zirconia having a lattice constant in a range from 99.95 to 99.97% compared with a lattice constant of a single crystal of stabilized zirconia is used as a compound in the filler. The stabilized zirconia may be calcia-stabilized zirconia having a lattice constant in a range from 5.1263 and 5.1275 Å.

(Note) Å: angstrom (10⁻¹ nm) is not the SI unit (The International System of Units), however since this is still customarily used often in the field of crystallography to which the invention is concerned, this is used as a unit of the lattice constant throughout the description.

As described above, change of the lattice constant of stabilized zirconia and that of the friction coefficient are in a close relation. The friction material containing stabilized zirconia having its lattice constant in a certain range changes its constant scarcely even after a friction of the friction material under a high load condition. Therefore, the friction coefficient of such a friction material also scarcely changes after the test. In the consequence stability of the friction coefficient at the initial stage of using a new friction material can be ensured by using the friction material of this invention.

Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relation between an annealing time and a lattice constant;

FIG. 2 is a graph showing the result of X-ray diffraction of calcia-stabilized zirconia;

FIG. 3 is a graph showing the result of X-ray diffraction of silicon (Si);

FIG. 4 is a graph showing experimental values and a peak position by theoretical calculation: an enlarged vicinity of a 311 peak of Si in FIG. 3;

FIG. 5 is a graph showing experimental values and a peak position by theoretical calculation: an enlarged vicinity of a 400 peak of Si in FIG. 3;

FIG. 6 is a graph showing experimental values and a peak position by theoretical calculation: an enlarged vicinity of a 331 peak of Si in FIG. 3;

FIG. 7 is a graph showing experimental values and a peak position by theoretical calculation: an enlarged vicinity of a 422 peak of Si in FIG. 3;

FIG. 8 is a graph showing experimental values and a peak position by theoretical calculation: an enlarged vicinity of a 511 peak of Si in FIG. 3;

FIG. 9 is a graph showing experimental values and a peak position by theoretical calculation: an enlarged vicinity of a 440 peak of Si in FIG. 3;

FIG. 10 is a graph showing experimental values and a peak position by theoretical calculation: an enlarged vicinity of a 531 peak of Si in FIG. 3;

FIG. 11 is a graph showing experimental values and a peak position by theoretical calculation: an enlarged vicinity of a 620 peak of Si in FIG. 3;

FIG. 12 is a graph showing a relation between the experimental peak positions and the theoretical ones determined from the eight peaks in FIGS. 4 to 11, and a correction formula of a systematic error, R: a correlation coefficient;

FIG. 13 is a graph showing experimental values before and after angle corrections: an enlarged 311 peak of the calcia-stabilized zirconia in FIG. 2;

FIG. 14 is a graph showing experimental values before and after angle corrections: an enlarged 222 peak of a calcia-stabilized zirconia in FIG. 2;

FIG. 15 is a graph showing experimental values before and after angle corrections: by enlarging 400 peak of the calcia-stabilized zirconia in FIG. 2;

FIG. 16 is a graph showing experimental values before and after angle corrections: an enlarged 331 peak of the calcia-stabilized zirconia in FIG. 2;

FIG. 17 is a graph showing experimental values before and after angle corrections: an enlarged 420 peak of the calcia-stabilized zirconia in FIG. 2;

FIG. 18 is a graph showing experimental values before and after angle corrections: an enlarged 422 peak of the calcia-stabilized zirconia in FIG. 2;

FIG. 19 is a graph showing experimental values before and after angle corrections: an enlarged 333 peak of the calcia-stabilized zirconia in FIG. 2;

FIG. 20 is a graph showing experimental values before and after angle corrections: an enlarged 440 peak of the calcia-stabilized zirconia in FIG. 2;

FIG. 21 is a graph showing experimental values before and after angle corrections: an enlarged 531 peak of the calcia-stabilized zirconia in FIG. 2;

FIG. 22 is a graph showing experimental values before and after angle corrections: an enlarged 442 peak of the calcia-stabilized zirconia in FIG. 2;

FIG. 23 is a graph showing experimental values before and after angle corrections: an enlarged 620 peak of the calcia-stabilized zirconia in FIG. 2;

FIG. 24 is a graph showing experimental values before and after angle corrections: an enlarged 533 peak of the calcia-stabilized zirconia in FIG. 2;

FIG. 25 is a graph showing the change of the lattice constant of calcia-stabilized zirconia after it is mixed with the other ingredients, and the friction material is formed: “single phase” stand for “before the formation”; “in friction materials”, “after the formation”;

FIG. 26 is a graph showing the change of friction coefficient after a high load friction test: the average friction coefficient during the second effectiveness test and that during the final inspection;

FIG. 27 is a graph showing the change of the lattice constant after a series of friction tests including the second effectiveness test and the final inspection;

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Exemplary embodiments of the invention will be described with reference to the accompanying drawings.

A friction material according to an exemplary embodiments of the present invention is provided with: a matrix fiber; a binder; and a filler, and a stabilized zirconia as one of abrasives in a portion of the filler. In the friction material, stabilized zirconia contained therein has a lattice constant in a range from 99.93 to 99.95% compared with a lattice constant of a single crystal of stabilized zirconia. The friction material containing stabilized zirconia of the lattice constant within the range described above can suppress the change of the friction coefficient even before and after the friction materials are used under the high load conditions.

So long as the friction material contains stabilized zirconia having the lattice constant in the range from 99.93 to 99.95% compared with the lattice constant of the single crystal of stabilized zirconia, the change of the lattice constant is small at the initial stage of using the friction material. Accordingly, a friction coefficient of the friction material is also stable. In a case where the value of the lattice constant of the stabilized zirconia is less than 99.93% compared with that of the single crystals of stabilized zirconia, distortion of the crystal is excessively large to partially form an amorphous phase and it no longer functions as the abrasives in the friction materials. In a case where the value of the lattice constant exceeds 99.95%, the lattice constant decreases greatly after the high load braking test and the friction coefficient changes before and after the test.

In a case of metal crystals, it has generally been known that the lattice constant decreases as the distortions are introduced and, correspondingly, the hardness increases. While the stabilized zirconia is a ceramic material, it is considered that the hardness increases as the lattice constant decreases also in this case. Deducing from this phenomenon, it is considered that the friction coefficient is not stable at the initial state of using the new brake products because the lattice constant decreases due to the introduction of distortions in the stabilized zirconia by friction under a high load and, correspondingly, the hardness of the stabilized zirconia increases. That is, it is estimated that distortions are introduced into the stabilized zirconia in the friction material by friction under a high load resulting in an increase of the hardness, and the friction coefficient also increases. Therefore, the friction coefficient at the initial stage of using the new friction materials is not stable. Also in the experiment of the present invention, it was confirmed that the lattice constant of the stabilized zirconia decreased after such a friction test.

In this invention, the single crystal of stabilized zirconia means a crystal with negligible lattice distortions. While the lattice constant, for example, in a case of calcia-stabilized zirconia is 5.1289 Å (“Inorganic Crystal Structure Database”, ICSD#60609), we have also confirmed from the following experiment that the lattice constant of the calcia-stabilized zirconia is 5.1289 Å.

The calcia-stabilized zirconia with distortion was annealed at 1300° C. and a relation between the annealing time and the lattice constant was examined. The result is shown in FIG. 1. As the annealing time becomes longer, the lattice constant gradually approaches 5.1289 Å. Generally, when thermal energy is given to a crystal by annealing, the lattice distortion energy is released, and the distortions become a negligible value. That is, ions of calcium, zirconium, and oxygen in the crystal lattice move into proper positions in the lattice and the internal energy of the calcia-stabilized zirconia crystal decreases. As the annealing time increase further, the internal energy becomes a minimum value, and the internal energy scarcely changes after that. That is, this is an crystal with no lattice distortions and the lattice constant thereof are those of a single crystal. In a case of calcia-stabilized zirconia, the lattice constant can be said to be 5.1289 Å also from the result of this measurement.

The stabilized zirconia is known as a substance formed by solution between a stabilizer such as calcia (CaO), yttria (Y₂O₃), magnesia (MgO) and a pure zirconia crystal to cause a crystal phase transition of zirconia which is a monoclinic crystal at room temperature into a cubic one to result in the cubic crystal phase stable enough even at the higher temperatures.

The stabilized ratio showing the degree of stabilization is determined by X-ray diffraction of a zirconia crystal and calculated according to the following equation,

Stabilized ratio (%)=I/(I+S)×100

Where I is the integrated intensity for the 111 diffraction peak of a cubic crystal (which appears at a diffraction angle in 2θ, between 28.90 and 30.90° when Cu K_(α) radiation having a wavelength of 1.540562 Å is used as an x-ray source, for example), and S, the addition of the integrated intensity for the −111 peak of a monoclinic crystal which appears at between 27.40° and 28.90°, and that of the 111 peak of the monoclinic crystal which appears at between 30.90° and 32.00°.

Further, zirconia in which a cubic and a monoclinic crystal coexist is referred to as a partially stabilized zirconia. The difference of the effect due to that of the stabilized ratio has not been observed as disclosed, for example, in JP-A-09-031440. Also in this invention, it is confirmed that the stable friction characteristics at the initial stage of using the brake can be obtained irrespective of the stabilized ratio. Therefore, the partially stabilized zirconia will be also referred simply to “stabilized zirconia”.

Generally, for the calculation of the lattice constant, a diffraction angle 2θ for each peak is measured at first and an interplanar spacing d is calculated according to the Bragg's equation: λ=2d·sin θ. In a case of a cubic crystal as in the stabilized zirconia, the lattice constant can be determined from the interplanar spacing d obtained for each of the peaks according to the relation between the lattice constant and each of the interplanar spacing as: lattice constant (Å)=λ/(2×sin θ)×(h²+k²+l²)^(0.5) Particularly, since the d value for the diffraction line obtained at a large angle is highly accurate, diffraction peaks at the higher angles are used for determining lattice constants at high accuracy. For example, in a case of determining the lattice constant of the calcia-stabilized zirconia by using Cu—Kα₁ line (wavelength λ=1.540562 Å), it is calculated according to the following theoretical formulae by using peak angles such as for a 311 peak where the diffraction angle (2θ) appears between 59.50° and 60.00°, for a 222 peak where it appears between 62.50° and 63.00°, and for a 400 peak where it appears between 73.50° and 74.00°.

In a case of calculation from 311 peak:

Lattice constant (Å)=λ/(2×sin θ)×11^(0.5)  (1)

In a case of calculation from 222 peak:

Lattice constant (Å)=λ/(2×sin θ)×12^(0.5)  (2)

In a case of calculation from 400 peak:

Lattice constant (Å)=λ/(2×sin θ)×16^(0.5)  (4)

In a case of calculation from 331 peak:

Lattice constant (Å)=λ/(2×sin θ)×19^(0.5)  (5)

In a case of calculation from 420 peak:

Lattice constant (Å)=λ/(2×sin θ)×20^(0.5)  (6)

In a case of calculation from 422 peak:

Lattice constant (Å)=λ/(2×sin θ)×24^(0.5)  (7)

In a case of calculation from 333 peak:

Lattice constant (Å)=λ/(2×sin θ)×27^(0.5)  (8)

In a case of calculation from 440 peak:

Lattice constant (Å)=λ/(2×sin θ)×32^(0.5)  (9)

In a case of calculation from 531 peak:

Lattice constant (Å)=λ/(2×sin θ)×35^(0.5)  (10)

In a case of calculation from 442 peak:

Lattice constant (Å)=λ/(2×sin θ)×36^(0.5)  (11)

In a case of calculation from 620 peak:

Lattice constant (Å)=λ/(2×sin θ)×40^(0.5)  (12)

In a case of calculation from 533 peak:

Lattice constant (Å)=λ/(2×sin θ)×43^(0.5)  (13)

Further, for calculating the lattice constant at higher accuracy, a systematic error has to be taken into consideration. The systematic error is introduced by a difference between a measured value and a theoretical value for the diffractoin angles caused by the breadth of X-ray beam or that of an incidence angle of X-ray to the specimen, etc., which depends on the configuration of x-ray optics of an X-ray diffractometer, specimen to be measured, etc. Generally, there are two typical way to correct such systematic errors: the internal standard method and the external standard method. In the internal standard method a standard substance whose lattice constant is previously determined are mixed into a specimen to be measured, and estimating the systematic errors based on the deviation of the diffraction angle of the standard specimen. To the contrary, in the external standard method the systematic errors are estimated by measuring the standard substance and the specimen to be measured separately. The lattice constant of the stabilized zirconia contained in the friction material is sometimes changed due to the introduction of the lattice distortions in a pulverization process. Such a pulverization process is indispensable when the internal standard method is applied to this case since the friction materials including the stabilized zirconia must be pulverized to be mixed with the powder of a standard substace. Accordingly, since in order to avoid to such artificial introduction of lattice distortions, it is necessary to measure the friction material in a solid state. Therefore, the external standard method is definitely more suitable than the internal one for the purpose. It is also preferred to employ the external standard method for measuring the lattice constant of the stabilized zirconia as the single substance before formation of friction materials.

For the standard specimen, a substance having stable lattice constant and its diffraction peaks not overlaping with those from the specimen must be selected. In the present case in which the lattice constant of the calcia-stabilized zirconia must be determined accurately, silicon (Si) is preferred as the standard specimen. FIGS. 2 to 24 show examples of the correction method. FIG. 2 is a result for X-ray diffraction of the calcia-stabilized zirconia and FIG. 3 is that for silicon (Si). FIGS. 4, 5, 6, - - - , show enlarged portions in the vicinity of the peak angles for 111, 220, 311, - - - , of silicon appeared in FIG. 3, respectively. FIG. 12 shows a relation between an experimental value (x-axis) and a theoretical value (y-axis). A regression formula in FIG. 12 is a correction formula for the systematic error. Correction values as shown in FIGS. 13 to 24 are obtained by substituting experimental values of the peak angles 311, 222, 400, - - - of calcia-stabilized zirconia for X in the formula. A lattice constant is obtained by substituting the corrected value into the formulae (1) to (13). By the method, the lattice constant of the stabilized zirconia can be determined at an accuracy in the order of 0.0001 Å.

Since about ten different kinds of crystal phases together with the stabilized zirconia are contained in the friction materials, there are portions where diffraction peaks for the stabilized zirconia and other crystal phases overlap with each other upon determining the lattice constant of the stabilized zirconia contained in the friction material. At the overlapped portion of the peaks, the peak positions of the stabilized zirconia can not be measured accurately. Accordingly, it is necessary to measure the peak positions of stabilized zirconia in a range of 2θ where the peaks do not overlap with other peaks. In a case of the stabilized zirconia in the friction materials, while peaks tend to overlap at the lower angles since peak distances are generally small, the lattice constant of the stabilized zirconia contained in the friction material can be determined accurately by using the peaks such as 331, 420, 333, 531 which exist at rather the higher angles in 2θ.

Then, production of the friction materials of the exemplary embodiment of the invention is to be described. The friction material according to the exemplary embodiment can be prepared by uniformly mixing a matrix fiber such as glass fiber or aramid fiber, a lubricant such as graphite or molybdenum disulfide, a friction modifier such as an organic friction dust, a metal powder and some inorganic materials, and a binder such as phenolic resin with the stabilized zirconia as described later and molding them under heating and pressure.

As described above, the friction materials of the exemplary embodiment is a friction material containing a stabilized zirconia having lattice constant in a range between 99.93 and 99.95% compared with single crystal of the stabilized zirconia. Accordingly, while any of stabilized zirconia may be used for the starting material so long as it has the lattice constant described above in the friction material, it is preferred to use, as the starting material, a stabilized zirconia having lattice constant in a range between 99.95 and 99.97% compared with the single crystal of stabilized zirconia. This is because the lattice constant of stabilized zirconia tends to decrease upon molding the friction material and it is considered that the lattice constant decreases upon molding because the lattice distortions increase by pressing upon molding.

The stabilized zirconia used in the exemplary embodiment is prepared, for example, by an electrofusing method and a subsequent stabilizing treatment of baddeleyte or zircon sand with a stabilizer such as calcia (CaO), yttria (Y₂O₃), or magnesia (MgO). By the treatment, the monoclinic zirconia is transformed into a cubic one, the structure of which is stable at a wide temperature range. Then, any values of the stabilized ratio will be acceptable since the change of the ratio gives little effect on the stability of the friction coefficient at the initial stage as described above. It has been known as described also in JP-A-09-031110, that the wear amount of the rotor, an opposing pair member for friction, increases remarkably in a case of partially stabilized zirconia with the stabilized ratio of less than 50%. Since the wear amount of the pair member increases depending on the hardness of the pair material in a case where the stabilized ratio is less than 50%, it is preferred that the stabilized ratio is 50% or more. However, in a case where the wear amount of the opposing pair member does not increase so much, using partially stabilized zirconia having the stabilized ratios of smaller than 50% will be acceptable for the puropose of the present invention. Further, the amount of addition of the stabilizer necessary for obtaining a 100% stabilized zirconia (consisting only of cubic crystals) is, for example, as described below.

Calcia (CaO): 4 to 8 mass %

Yttria (Y₂O₃): 6 to 10 mass %

Magnesia (MgO): 4 to 8 mass %

To control the lattice constant of stabilized zirconia to the values between 99.95 and 99.97% of those of the single crystal of the stabilized zirconia, there are various methods including, for example, pulverization. Generally, it has been known that distortions introduced into the crystals decrease their lattice constant in the course of pulverization etc. Therefore, also in the case of stabilized zirconia, the lattice constant can be decreased by pulverization. Any methods other than that described above can be also used in the invention so long as the lattice constant can be controlled to the values between 99.95 and 99.97% of those of the single crystals of the stabilized zirconia. As will be shown in the Example section, It has been found that the friction characteristics aimed in the invention, that is, the stability of the friction coefficient at the initial stage of using the new friction materials does not depend on the particle size and the shape of stabilized zirconia. However, when other friction characteristics, for example, wear resistance and the subsequent average friction coefficient must be considered together with the stability of the friction coefficient in the initial stage as the aim of the invention, it is preferred that the average particle size is from several micron (μm) to several tens micron (μm) and the shape is of an acute angle with surface area/volume being as large as possible.

While the amount of the stabilized zirconia together with other filler ingredients in friction materials should be changed and determined depending on the values of the friction coefficient required, an excessively amount of that is not preferred since this increases a wear rate of a friction pair such as rotor. On the other hand, if the zirconia content is insufficient, it does not function as abrasives. Accordingly, it is preferably within a range from 0.5 to 25% by weight based on the total amount of the friction material.

As the abrasives ingredient used in the friction material according to the exemplary embodiment, abrasives used so far, for example, silica and alumina can be used together in addition to stabilized zirconia.

Ingredients other than the abrasives may be identical with existent materials and, for example, a resin based on phenol resin or melamine resin, or modified products thereof can be used.

As the fiber matrix, organic or inorganic fibers comprising, for example, copper, stainless steel, brass, aramid, carbon, glass, potassium titanate, rock wool, and ceramics can be used.

As the fillers other than the abrasives, a body filler such as barium sulfate and calcium carbonate, a solid lubricant such as graphite and molybdenum disulfide, organic dust such as cashew dust and rubber dust, metal powder such as iron, copper, and aluminum can be used.

After stirring and mixing the blended composition described above thoroughly and preliminarily molding of them, for example, into tablet at an uniaxial pressure of 100 to 500 Kgf/cm² (9.8 to 49.0 MPa), it is placed together with a backing plate in a hot press, and then thermally formed at a temperature of 130 to 180° C., under an uniaxial pressure of 200 to 1,000 Kgf/cm² (19.6 to 98.0 MPa) for about 3 to 15 min, and then the molding product is heat treated at a temperature from 150 to 300° C. for about 1 to 15 hours and, further, applied with shape fabrication to obtain a friction material of the exemplary embodiment of the invention.

EXAMPLE

The present invention is to be described by way of examples in a case of using calcia-stabilized zirconia, another stabilized zirconia phase stabilized with yttria or magnesia, however, can also be used in the same manner and the invention is not restricted to the following examples. Table 1 shows the blending ratio (formulation)(wt %) of the ingredients of the friction material used in the examples of the invention, the lattice constant of the stabilized zirconia as a single phase used in the example, the stabilized ratio, and the annealing time. The average particle size of the calcia-stabilized zirconia used was several μm to several 10 μm.

Examples 1 to 6 <Preparation of Calcia-Stabilized Zirconia>

The calcia-stabilized zirconia used in Examples 1 to 6 was prepared by an electrofusing method. Baddeleyte and calcia were mixed, electrically heated to melt the mixture thereof and cooled and solidified. When the calcia-stabilized zirconia were pulverized for a certain time and measured for the X-ray diffraction, the lattice constant was 5.1260 Å. Then, by annealing them for the time shown in Table 1 respectively, stabilized zirconia having respective lattice constant could be obtained (refer to FIG. 1).

<Production of Friction Material>

After sufficiently stirring and mixing the calcia-stabilized zirconia prepared as described above with other starting materials at blending ratios shown in Table 1 and molding them into tablets at an uniaxial pressure of 200 Kgf/cm² (19.6 MPa) preliminarily, this preliminary molded product was placed together with a backing plate to a hot press and thermally formed at a temperature of 150° C. under an uniaxial pressure of 400 Kgf/cm² (39.2 MPa) for 10 min. Then, the molded product was heat treated at a temperature of 250° C. for 8 hours and cut into a shape of a brake pad.

Comparative Examples 1 to 4 <Preparation of Calcia-Stabilized Zirconia>

Calcia-stabilized zirconia of Comparative Examples 1 to 4 were also prepared by the same method as in the examples. The particle size of the calcia-stabilized zirconia used was identical with those in the examples.

<Production of Friction Material>

After uniformly mixing and stirring the calcia-stabilized zirconia prepared as described above with other starting materials at blending ratios shown in Table 1, friction materials were prepared in the same manner as in examples.

TABLE 1 Ingredients Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Formulation (Blending ratio) (wt %) Phenol resin 10 10 10 10 10 10 Resin dust 5 5 5 5 5 5 Rubber powder 5 5 5 5 5 5 Barium sulfate (BaSO₄) 20 20 20 20 25 10 Graphite 10 10 10 10 10 10 Magnetite (Fe₃O₄) 5 5 5 5 5 5 Potassium titanate 15 15 15 15 15 15 fiber Copper fiber 15 15 15 15 15 15 Aramid fiber 5 5 5 5 5 5 Calcia-stabilized 10 10 10 10 5 20 zirconia Lattice constants of calcia-stabilized zirconia having various stabilized ratios and anneling times (values as a single phase before forming into friction materials) Lattice constant (Å) 5.1265 5.1271 5.1267 5.1268 5.1270 5.1275 Stabilized ratio (%) 100 100 70 40 100 100 Annealing time 28 74 42 50 78 100 (hour) Comp. Comp. Comp. Comp. Ingredients Example 1 Example 2 Example 3 Example 4 Formulation (Blending ratio) (wt %) Phenol resin 10 10 10 10 Resin dust 5 5 5 5 Rubber powder 5 5 5 5 Barium sulfate (BaSO₄) 20 20 20 20 Graphite 10 10 10 10 Magnetite (Fe₃O₄) 5 5 5 5 Potassium titanate fiber 15 15 15 15 Copper fiber 15 15 15 15 Aramid fiber 5 5 5 5 Calcia-stabilized zirconia 10 10 10 10 Lattice constants of calcia-stabilized zirconia having various stabilized ratios and anneling times (values as a single phase before forming into friction materials) Lattice constant (Å) 5.1286 5.1279 5.1283 5.1289 Stabilized ratio (%) 100 100 70 40 Annealing time (hour) 65 50 60 73

The lattice constant of the calcia-stabilized zirconia contained in the friction materials was measured by the method shown for the calculation methods for the lattice constant described above. The brake pad was cut into 35 mm in length×35 mm in width×20 mm in thickness which was used as samples.

The measuring conditions were as shown below.

-   X-ray source: Cu target, -   X-ray wavelength (Cu—Kα₁ ray): 1.540562 Å (monochromatic X-ray), -   X-ray source power (voltage/current): 40 kV/40 mA, -   slits sizes: DS:1° SS:1° RS: 0.15 mm, -   where DS: Divergence slits, SS: Scattering slits, and RS: Receiving     slits, -   Scan method: step scan, -   2θ step width: 0.02°, -   Measuring angle: 2θ 80° to 130°, -   Counting (dwelling) time for 10 sec/step, -   sample rotated within the sample surface plane.

Silicon (Si) at a purity of 98% up manufactured by High Purity Chemical Co. was used for the standard specimen for the angle correctoins. The results are shown in Table 2 and FIG. 25.

TABLE 2 Lattice constant: Å (ratio to the Examples & value of the single crystal: %) Comparative Values as a single Values in friction Stabilized ratio Examples phase materials (%) Example 1 5.1265 (99.95) 5.1253 (99.93) 100 Example 2 5.1271 (99.96) 5.1259 (99.94) 100 Example 3 5.1267 (99.96) 5.1255 (99.93) 70 Example 4 5.1268 (99.96) 5.1256 (99.94) 40 Example 5 5.1270 (99.96) 5.1258 (99.94) 100 Example 6 5.1275 (99.97) 5.1263 (99.95) 100 Comp. Example 1 5.1286 (99.99) 5.1274 (99.97) 100 Comp. Example 2 5.1279 (99.98) 5.1267 (99.96) 100 Comp. Example 3 5.1283 (99.99) 5.1271 (99.96) 70 Comp. Example 4 5.1289 (100.0) 5.1277 (99.98) 40

In both of the examples and the comparative examples, the lattice constant decreased by about 0.02% after molding into a friction material.

<Friction Test>

For measuring the range of the friction coefficient before and after a high load braking, a test was conducted to the friction materials of the examples and the comparative examples by a brake dynamometer under the following conditions.

An evaluation test was conducted under the conditions according to JASO C 406-2000 with various brake factors. Brake caliper: 1 pot type of 57.15 mm wheel cylinder diameter, effective tire radius: 306 mm, rotor size: φ 255 mm, and rotor material: FC200, and pad friction area: 43 cm². For the friction coefficient before high load braking, a friction coefficient in braking at a brake initial speed: V₀=100 km/h, deceleration α=2.94 M/s² was used as a representative value in a second effectiveness test. Further, for the friction coefficient after the high load braking, a friction coefficient in braking at V₀=100 km/h, and α=2.94 M/s² in the final effectiveness test after the first and the second fade tests was used as a representative value. The test result is shown in Table 3 and FIG. 26.

TABLE 3 Friction coefficient (V₀ = 100 km/h, α = 2.94 m/s²) (Comparative) Second Examples effectiveness test Final inspection Example 1 0.435 0.433 Example 2 0.441 0.433 Example 3 0.429 0.427 Example 4 0.415 0.418 Example 5 0.391 0.389 Example 6 0.459 0.468 Comp. Example 1 0.385 0.423 Comp. Example 2 0.397 0.455 Comp. Example 3 0.378 0.421 Comp. Example 4 0.354 0.403

<Measurement for Lattice Constant Before and after Friction Test>

For friction materials of Examples 1 to 6 and Comparative Examples 1 to 4, the lattice constants of the calcia-stabilized zirconia in the friction materials before and after the friction test were measured by the method of calculating the lattice constant using an X-ray diffractometer under the same conditions as in the test shown in Table 2.

Table 4 and FIG. 27 show the results of measurement.

TABLE 4 Lattice constant: Å (ratio to single crystal: %) Before friction test After friction test Stabilized ratio Example 1 5.1253 (99.93) 5.1253 (99.93) 100 Example 2 5.1259 (99.94) 5.1259 (99.94) 100 Example 3 5.1255 (99.93) 5.1255 (99.93) 70 Example 4 5.1256 (99.94) 5.1256 (99.94) 40 Example 5 5.1258 (99.94) 5.1258 (99.94) 100 Example 6 5.1263 (99.95) 5.1263 (99.95) 100 Comp. Example 1 5.1274 (99.97) 5.1266 (99.96) 100 Comp. Example 2 5.1267 (99.96) 5.1264 (99.95) 100 Comp. Example 3 5.1271 (99.96) 5.1265 (99.95) 70 Comp. Example 4 5.1277 (99.98) 5.1267 (99.96) 40

In the friction materials of Examples 1 to 6 of the invention, the friction coefficient changed little and also the lattice constant scarcely changed before and after the high load braking.

On the other hand, in the friction materials of Comparative Examples 1 to 4, the friction coefficient increased greatly before and after the thermal hysteresis at the high load. Further, the lattice constant decreased after the thermal hysteresis at the high load. Further, the friction materials containing the stabilized zirconia with the lattice constant of less than 5.1263 Å could not endure usual use as a brake.

According to the exemplary embodiments, since the friction material ensuring the stability of the friction coefficient at the initial stage of using the brake can be obtained, the friction material and the production method thereof of the invention are useful in the field of producing the friction materials.

It will be apparent to those skilled in the art that various modifications and variations can be made to the described exemplary embodiments of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover all modifications and variations of this invention consistent with the scope of the appended claims and their equivalents. 

1. A friction material comprising: a matrix fiber; a binder; a filler; and a stabilized zirconia having a lattice constant in a range from 99.93 to 99.95% as compared with a lattice constant of a single crystal of stabilized zirconia.
 2. The friction material according to claim 1, wherein the stabilized zirconia is a calcia-stabilized zirconia having a lattice constant in a range from 5.1253 to 5.1263 Å.
 3. A friction material comprising: a matrix fiber; a binder; and a filler, wherein a stabilized zirconia having a lattice constant in a range from 99.95 to 99.97% as compared with a lattice constant of a single crystal of stabilized zirconia is utilized as a compound in the filler.
 4. The friction material according to claim 3, wherein the stabilized zirconia is a calcia-stabilized zirconia having a lattice constant in a range from 5.1263 to 5.1275 Å.
 5. A method of manufacturing a friction material comprising: uniformly mixing a matrix fiber, a binder, and a filler; and molding a mixture of the matrix fiber, the binder, and the filler under heating and pressure, wherein a stabilized zirconia having a lattice constant in a range from 99.95 to 99.97% as compared with a lattice constant of a single crystal of stabilized zirconia is utilized as a compound in the filler.
 6. The method according to claim 5, wherein the stabilized zirconia is a calcia-stabilized zirconia having a lattice constant in a range from 5.1263 to 5.1275 Å. 